1. Field of the Invention
Embodiments of the present invention relate to storage devices. In particular, embodiments of the present invention relate to an on-slider shunt resistor with a low thermal coefficient of resistivity (TCR) for reducing the impedance mismatch between a tunneling magneto-resistive (TuMR) read sensor, a transmission line, and a preamplifier while preserving the ability to measure the TCR of the TuMR read sensor accurately to eliminate defective TuMR read sensors.
2. Related Art
A major goal among many disk drive manufacturers is to continue to increase an amount of data that can be stored on a recording medium while still maintaining data integrity and disk drive reliability. Likewise, another major goal among many disk drive manufacturers is to continue to increase the rate at which disk drives read data from and write data to a recording medium while likewise maintaining data integrity and disk drive reliability.
When data is read, a magneto-resistive (MR) element exhibits a change in electrical resistance as a function of external magnetic field. In storage systems, a magnetic bit on the disk modulates the resistance of the MR element as the bit passes below. The change in resistance can be detected by passing a sense current through the MR element and measuring the voltage across the MR element. The disk drive uses the resultant signal to recover data from the magnetic storage medium. Depending on the structure of a device, the MR effect can fall into different categories, such as anomalous MR, giant MR (GMR), and tunneling MR (TuMR). The use of the latter has been the latest technique to increase the rate at which disk drives read data.
However, increasing the rate at which data is read can be problematic. In order to increase the data transmission rate, the frequency of the signal must be increased accordingly. However, a signal using a high frequency can be subject to distortion, frequency dependent gain, noise increase, and phase shifting.
Perhaps most problematic for TuMR devices is a reduction in high frequency capability of a TuMR read sensor due to a high resistance at the TuMR read sensor. Moreover the increased resistance may correspond to an impedance mismatch between the read structure containing the TuMR read sensor, a transmission line, and a preamplifier. In an ideal system, the impedance of the read structure, the transmission line, and the preamplifier match, or are about equal, thus allowing a signal or pulse to move from the TuMR read sensor along the transmission line to the preamplifier with minimal distortion due to frequency dependent gain and time delay. For example, FIG. 7 illustrates a GMR device, where the impedance (ZRSTR) of the read structure 41A may be about 50Ω, while the impedance (ZPA) of the preamplifier 38A and the impedance (ZTL) of the transmission line 34A may be about 60Ω. As such, the read structure 41A can transmit the signal to the preamplifier 38A via the transmission line 34A with limited distortion that would result from frequency dependent insertion loss if ZRSTR was very much larger than ZTL or ZPA. This is because the purpose of a transmission line is to convey electrical energy from one point to another, so the ideal situation would be for all of the original signal energy to travel from the source to the load and then be completely absorbed or dissipated by the load for maximum signal-to-noise ratio. As a result, when the impedances match, the preamplifier 38A is able to absorb the transmitted signal 100A in its entirety.
However, FIG. 8 illustrates a conventional TuMR device, where an impedance of a read structure 41B, a transmission line 34B, and a preamplifier 38B do not match due to a very high resistance of a TuMR read sensor within the read structure 41B. As is evident, the read structure 41B of a TuMR device has an impedance (300Ω or more) that is tremendously larger than the impedance of the preamplifier 38B (about 150Ω) or the transmission line 34B (˜75Ω). In this example the impedance of the preamplifier is set at an intermediate value between that of the TuMR and the transmission line 34B in order to achieve an engineering compromise between insertion loss distortion due to the higher TuMR resistance and distortion due to reflection of high frequency components 120 at the mismatched junction between the transmission line 34B and the preamplifier 38B. As a result, the preamplifier 38B will not be able to absorb the low and high frequency components of an incoming pulse 100B (i.e., data read from the recording medium by the TuMR read sensor) with the same efficiency and time delay, causing distortion. In addition, the TuMR read sensor will not be able to absorb the high frequency noise from the preamp which will echo along the transmission line 34B. As expected, the pulse distortion and extra noise degrades the reliability and integrity of the transmitted data, and essentially renders the desired high data rates of TuMR devices less useful.
There have been several proposals to reduce the high resistance of the TuMR read sensor and the corresponding impedance mismatch between the read structure, transmission line, and the preamplifier. For example, using a thin tunneling layer, which is a layer of insulating material located between two ferromagnetic layers within the TuMR read sensor, or a tunneling layer made of TiOx (instead of AIOx) have been suggested to reduce the TuMR read sensor resistance. However, thinning the tunneling layer beyond a critical point radically reduces the sensor response (e.g., ΔR/R) of the TuMR read sensor and results in reliability problems due to the tunneling layer having nascent pinholes and actual pinholes, which are nano-sized conductive shorts that form in between the two ferromagnetic layers within the TuMR read sensor. Pinholes allow a parasitic current to pass from one ferromagnetic layer to the other, which affects the magneto-resistance of the device, and ultimately the reliability of the device. Likewise, composing the tunneling layer of TiOx inadequately reduces the TuMR read sensor resistance required for high data transmissions for narrow read widths.
In addition, using a long stripe height of the TuMR read sensor has been proposed to reduce the total resistance of the read structure because resistance is inversely related to the area of resistance of the tunneling layer within the TuMR read sensor. Thus, as area increases due to a longer stripe height, resistance of the tunneling layer decreases accordingly. However, using a long stripe height has proven to be equally problematic since a longer stripe height can add shot noise, increase Thermal Magnetic Noise, and produce localized heating.
Furthermore, using a resistor at the preamplifier had been proposed to prevent a preamplifier designed to deliver a bias current to a low (˜50Ω) resistance GMR read sensor from damaging a more sensitive TuMR read sensor with a higher resistance (˜400Ω). The shunt resistor located at the preamplifier provides an alternate path for damaging bias currents, as most bias current are greater than 2 mA, while the TuMR read sensor damage threshold is less than 1 mA. However, this technique only reduces the impedance at the preamplifier, thus leaving the high resistance of the read structure unchanged. As a result, the impedance mismatch continued to exist.
In addition, previous shunt resistor applications, such as U.S. Pat. No. 7,054,085 entitled “Use of Shunt Resistor with Large RA Product Tunnel Barriers,” did not deal with the effect of the shunt resistor on a measurement of a TuMR read sensor thermal coefficient of resistivity (TCR). TCR is a measure of the slope of the resistance versus temperature (normalized to room temperature), and is useful for testing for defective TuMR read sensors. Without measuring the TCR of the TuMR read sensor, it is difficult to distinguish a defective TuMR read sensor from the norm. An ideal TuMR read sensor has a specific TCR, whereas a TuMR read sensor containing pinholes or nascent pinholes will have a differing TCR. Therefore, measuring the TCR of the TuMR read sensor can determine whether the TuMR read sensor is defective due to the presence of pinholes. However, this task can be difficult, if not impossible, if, for example, the read structure contains a shunt resistor with a large TCR because such a resistor would impact the calculation of the TCR of the TuMR read sensor, thus preventing accurate testing.
In light of the above-mentioned problems, there is a significant need for reducing the high resistance of the TuMR read sensor and the corresponding impedance mismatch between the read structure, the transmission line, and the preamplifier while preserving the ability to accurately measure the TCR of the TuMR read sensor to detect defective parts efficiently.